by Blake Moore
Humankind has been obsessed with circles for a long time. It comes as no surprise then that the modeling of gravitational waves had focused until recently on those emitted by black holes or neutron stars in circular orbits around each other. But in the case of gravitational wave modeling, there is good reason for this obsession. Gravitational waves remove energy and angular momentum from a binary, forcing the eccentricity to decay and the orbit to circularize rapidly. Since the 1960s, the expectation has then been that the gravitational waves that ground-based detectors would observe would correspond to circular binaries.
Travis Robson (right), Blake Moore (center), and David Anderson (left) are members of the eXtreme Gravity Institute at Montana State University. Here they are at nearby Yellowstone National Park.
But as with most things in physics, Nature adores the complex if one looks closely enough. Several astrophysical studies have recently shown that binaries may form with moderate eccentricities at orbital separations at which they would be emitting gravitational waves that ground-based detectors could observe very soon. These binaries would form near a supermassive black hole or in globular clusters, where three- or many-body interactions may source eccentricity. And if they are detected, they could shed light on their true population and formation scenario.
Can you tell us a little bit about the work in your thesis?
One of the challenges of modern cosmology is to interpret observations in a consistent and model-independent way. There are several assumptions in interpreting cosmological/astrophysical data. For example, it is often assumed that Einstein’s theory of gravity is the correct theory of gravity. Furthermore, fundamental to cosmology is the assumption that the universe is homogeneous and isotropic on the largest scales and hence, this is the correct starting point to interpret cosmological data. To test these assumptions, approaches are needed, which work in a model-independent way. Broadly speaking, my thesis addresses these questions.
Thinking back, what was the most interesting thing that happened during your PhD?
Dr Viraj A A Sanghai is a postdoctoral fellow working on theoretical cosmology at Dalhousie University in Halifax, Canada
The most interesting thing that happened during my PhD was the discovery of gravitational waves by LIGO, due to the merging of two black holes. This opened up a new avenue into testing Einstein’s theory of gravity and started the new field of gravitational wave astronomy. Before this, all our astronomical observations relied on electromagnetic radiation. This discovery is helping us to have a deeper understanding of our Universe. Consequently, the Nobel Prize in Physics was awarded for this discovery. Continue reading
Scott Melville, winner of the Best Student Talk Prize at BritGrav, which was sponsored by CQG, discusses the research that he’s doing on quantum gravity at Imperial College London.
Scott Melville, speaking at Bright Club on 28th April 2018. Image courtesy of Steve Cross
The present state of quantum gravity is rather unsatisfying. While perturbation theory works well at low energies, at high energies quantum gravity becomes incalculable, and leaves us hungry for answers. As we approach the Planck scale, perturbations become strongly coupled and we quickly lose perturbative control of our theory. A UV complete theory of gravity, which remains unitary and sensible to arbitrarily high energies, is hard to cook up.
We need new physics, to swallow these Planck-sized problems. This new physics shouldn’t be too heavy, or too light; not too strongly coupled, or too perturbative. We don’t yet know exactly what it should be, but it needs to hit a sweet spot. My research develops tools, called positivity bounds, which can help us better understand how low energy observables are connected to this unknown new physics.
One thing is for certain: quantum gravity is hard – and working on it sure builds up an appetite. When I’m not worrying about the fundamental nature of the Universe: I’m in the kitchen. While I may not be the best chef in the world, I make up for an abysmal lack of skill with a towering surplus of enthusiasm. You can flip anything in a pan, if you flip hard enough.
When it comes to deciding what to have for dinner, I take things very seriously: it can’t be too salty, or too sweet; not too spicy, or too bland.
by Dr. Donald G. Bruns
Don Bruns and his wife Carol on eclipse day at the Lions Camp on Casper Mtn. The tripod is bolted to the custom mosaic designed and built by his cousin Steve Lang.
After much anticipation, two experiments had great successes last year. On August 17 2017, the LIGO/VIRGO collaboration monitored the merger of two neutron stars millions of light years away. Only four days later in Wyoming, an experiment to measure the gravitational bending of starlight by the Sun acquired the best data since the idea was first tested in 1919, by Sir Arthur Eddington, in Africa. I published my results on that experiment in Classical and Quantum Gravity on March 6, 2018. My solo project to repeat Eddington’s achievement, which made Einstein famous, required a lot less manpower than LIGO!
Early last century, Einstein published his General Theory of Relativity that contained some unusual predictions, including the idea that massive bodies bend light beams. The only way to test this would be during a total eclipse, when the sky would be dark enough to see stars close to the Sun, where the effect just might be measurable.
I started planning Eddington’s re-enactment when I found out that no one had attempted it since 1973 (also in Africa) and that no one had ever succeeded in getting all the parts to work during those precious few minutes of totality. I assumed that with modern charge-coupled device (CCD) cameras and computerized telescopes, the experiment would be much easier. I was wrong! While some aspects were simplified (the Gaia star catalog provided accurate star positions, for example, and modern weather predictions and the compact equipment eased many logistics problems), dealing with pixels, turbulence, and a limited sensor dynamic range presented new challenges.
by Clifford Will, Editor-in-Chief, Classical and Quantum Gravity
The gravitational physics community, indeed the whole world, mourns the passing on Wednesday 14th March, 2018, of Stephen Hawking at the age of 76. The Editor, Board and staff of CQG offer their heartfelt condolences to Stephen’s family. There are already numerous extended obituaries of Stephen, and I won’t attempt one here (see for example the fine obituaries by Dennis Overbye in the New York Times and by Roger Penrose in the Guardian).
I will, however, offer two personal remembrances of Stephen that I hope will illustrate his humorous side. In 1972, I was a student at the famous Les Houches Summer School on black holes, where Stephen, Brandon Carter and Jim Bardeen lectured and wrote the seminal paper “The Four Laws of Black Hole Mechanics”, that suggested a formal analogy with the laws of thermodynamics. This was soon followed by papers by Jacob Bekenstein and by Stephen that made this more than an analogy. But one of the things I most remember about the school was the awe-struck look on my eight-year-old daughter Betsy’s face watching Stephen in his wheelchair demonstrating how he could wiggle his ears like Dumbo the elephant.
The second remembrance was a visit to Cambridge in 1978, where Stephen had asked me to give a colloquium on tests of GR and invited me and my wife to join him and Jane at “high table” dinner at his college, Gonville and Caius. I showed up in a psychedelic paisley shirt with ridiculously wide collars, baby blue flared jeans, and high-heeled boots (think John Travolta in “Saturday Night Fever”, but with hippie length hair). This was attire totally inappropriate for high table (hey, this was the 70s and was the best I had in my suitcase), but Stephen was delighted to have somebody there who made the stuffy and decorum-obsessed masters of the college more uncomfortable than he did. And when, during the ritual passing of the after-dinner liqueurs along the table, the college master chided me sternly for allowing the port to precede the claret, I thought Stephen was going slide out of his wheelchair, hysterical with laughter.
We have lost a remarkable scientist and a unique human being.
This work is licensed under a Creative Commons Attribution 3.0 Unported License.
It’s sophomore year of our Classical and Quantum Gravity reviewer of the year awards. This year congratulations go to Dr Matthew Pitkin whose reviews were not only of exceptional quality but also submitted in good time. Matt has dedicated even more time to CQG by answering these questions. Congratulations Matt!
Tell us how you go about reviewing an article?
I’d probably echo many of last years’ winners points. Firstly, I have to decide whether I think I have the expertise to review the article. Working in the field of gravitational waves, I quite often receive requests to review papers on aspects of theoretical gravity, which I have absolutely no relevant knowledge of. (Going by my day-to-day work I’m really just a self-taught software developer and data scientist, who masquerades as an astrophysicist!) If I decide that I am qualified, then I give the article a quick skim, print it out, write “For review” in big red letters on it, and sit it somewhere prominently on my desk, so that I can’t ignore it. I also set an online calendar reminder with the deadline for returning the review.
I normally actually sit down to perform the review during a lull in my day-to-day work, like when I’ve just set an analysis code running. I just go through it methodically with a red pen in hand and scribble on the print out when I hit things I don’t understand or think might be problematic. Often, I’ll find that parts I don’t understand are actually explained later on in the paper, so this can indicate that some rearrangement of the article might be in order to clarify things. I check for any stand-out mathematical errors, but don’t have the ability to check all derivations in papers. I try not to make comments for the sake of writing something if there aren’t any problems with the paper. When I do make comments, I try to give constructive advice about how to improve the clarity of the article, or where more explanation might be required. But, I also know that it’s not my job to re-write the article, so don’t give very lengthy comments or suggestions.
By Parampreet Singh, Louisiana State University, USA
A successful union of Einstein’s general relativity and quantum theory is one of the most fundamental problems of theoretical physics. Though a final theory of quantum gravity is not yet available, its lessons and techniques can already be used to understand quantization of various spacetimes. Of these, cosmological spacetimes are of special interest. They provide a simpler yet a non-trivial and a highly rich setting to explore detailed implications of quantum gravitational theories. Various conceptual and technical difficulties encountered in understanding quantum dynamics of spacetime in quantum gravity can be bypassed in such a setting. Further, valuable lessons can be learned for the quantization of more general spacetimes.
In the last decade, progress in loop quantum gravity has provided avenues which allow us to reliably answer various interesting questions about the quantum dynamics of spacetime in the cosmological setting. Quantum gravitational dynamics of cosmological spacetimes obtained using techniques of loop quantum gravity leads to a novel picture where singularities of Einstein’s theory of general relativity are overcome and a new window opens to test loop quantum gravity effects through astronomical observations.
The scope of the Focus Issue: Applications of loop quantum gravity to cosmology, published last year in CQG, is to provide a snapshot of some of the rigorous and novel results on this research frontier in the cosmological setting.
by Clifford M. Will, CQG Editor-in-Chief
What a week for gravitational physics!
First came the September 27th announcement of another detection of gravitational waves, this time by the three-detector network that included Virgo along with the two LIGO observatories. The source of the gravitational waves was another fairly massive black hole binary merger, with black holes of 30 and 26 solar masses. Once again, about 3 solar masses were converted to energy in a fraction of a second, leaving behind a 53 solar mass black hole spinning at about 70 percent of the maximum allowed. With Virgo included in the detection, the localization of the source on the sky was improved dramatically over earlier detections by LIGO alone, dropping to a small blob on the sky measuring 60 square degrees, from the large, 1000 square degree banana-shaped regions of earlier detections.
For the first time, a test of gravitational-wave polarizations was carried out. Because the arms of the two LIGO instruments are roughly parallel, they have very weak sensitivity to different polarization modes of the waves. But with Virgo’s very different orientation, it was possible to show that the data favor the two spin-2 modes of general relativity over pure spin-0 or pure spin-1 modes.
But then, six days later came the announcement of the Nobel Prize in Physics, awarding one half of the prize to Rainer Weiss of MIT and the other half shared between Kip Thorne and Barry Barish of Caltech, for decisive contributions to the detection of gravitational radiation. CQG congratulates the winners!
by Ivan Agullo, Abhay Ashtekar and Brajesh Gupt
Can observations determine the quantum state of the very early Universe?
Can we hope to know even in principle what the universe was like in the beginning? This ancient metaphysical question has acquired new dimensions through recent advances in cosmology on both observational and theoretical fronts. To the past of the surface of last scattering, the universe is optically opaque. Yet, theoretical advances inform us that dynamics of the universe during earlier epochs leaves specific imprints on the cosmic microwave background (CMB). Therefore, we can hope to deduce what the state of the universe was during those epochs. In particular, success of the inflationary scenario suggests that the universe is well described by a spatially flat Friedmann, Lemaître, Robertson, Walker (FLRW) space-time, all the way back to the onset of the slow roll phase. This is an astonishingly early time when space-time curvature was some times that on the horizon of a solar mass black hole and matter density was only 11 orders of magnitude smaller than the Planck scale.
Clockwise from top left: Brajesh Gupt (Pennsylvania State University), Abhay Ashtekar (Pennsylvania State University) and Ivan Agullo (Louisiana State University)
It’s been a busy few weeks for CQG – we’ve been to the Era of Gravitational Wave Astronomy conference in Paris, hosted the annual Editorial Board meeting in London, attended the Loops17 conference in Warsaw and now it’s time to fly off to California for Amaldi12.
Amaldi12, named after Edoardo Amaldi, will be held at the Hilton Hotel in Pasadena, CA from 9th – 14th July. The conference will explore the science around gravitational waves and their detection, particularly in light of the confirmed detections by LIGO-Virgo and new advances with the LISA mission.
I will be at the conference Monday through Friday with a table top booth at the event, located near the international ballroom in the hotel. I’m really interested in hearing your thoughts about the journal, so please do stop by say hello and have a chat.